The two drug categories that will be focused on are Selective Serotonin Inhibitors (SSRIs) and Fluoroquinolones.

Pharmacodynamics describes how the body reacts to the drug administered. Selective Serotonin Inhibitors is a class of antidepressants. SSRIs “inhibit the serotonin reuptake pump and increase postsynaptic serotonin receptor occupancy” which increase serotonergic activity (Hirsch & Birnbaum, 2020). SSRIs are very selective to serotonin receptors, but this class of drugs takes weeks for full therapeutic effects (Hirsch & Birnbaum, 2020). There are many factors that can affect the pharmacodynamics of SSRIs including genetics and other drug interactions. One example of a medication that interacts with SSRIs is St. John’s Wart. If an SSRI and St. John’s Wart are combined it can create a sedative effect (Gordon, 1998). If there is too much of an SSRI in the body or if titration of the drug happens too quickly, serotonin syndrome is also possible which could cause GI upset, mental status changes and autonomic dysfunction (Woo & Robinson, 2016).

Fluoroquinolones is an antibiotic class that is used to target aerobic or gram-negative bacilli by inhibiting bacterial DNA synthesis (Lode, Borner, Koeppe, 1998) (Hooper, 2019). With the correct doses of fluoroquinolone, studies have shown a reduction in bacteria causing infections. In fact, one study using animal and patient models showed that “regimens of large doses given at infrequent intervals might be most efficacious in terms of bacterial killing, eradication time, and reducing the selection of resistant bacteria” (Lode, Borner, Koeppe, 1998). A factor that could influence how the body reacts to this antibiotic class is environment. Fluoroquinolones are becoming less effective against certain drug-resistant microbes, so depending on the bacteria causing the infection, fluoroquinolones might not be effective at eradicating the bacteria. Fluoroquinolones also have many adverse reactions including but not limited to GI upset, increased risk for C.difficile, and hepatic transaminases (Hooper, 2019).

Pharmacokinetics describes the process of what the body does with the drug once it has been administered. SSRIs are absorbed through the GI tract and then distributed through the body and brain after binding to proteins due to the lipophilic characteristic of SSRIs. SSRIs are then metabolized and excreted through the liver. The half-life for SSRIs is approximately one day (Hirsch & Birnbaum, 2020). Factors that can affect the pharmacokinetics of SSRIs include age, weight and liver function. For an example, if an elderly patient is started on an SSRI, they must start at a lower dose due to the effects of aging on the liver. In adults aged 65 or older, there is “an approximately 35% decrease in the blood volume of the liver compared with those aged less than 40 years” (Kim, Kisseleva & Brenner, 2015). With a decrease in blood flow to the liver, an elderly patient will not be able to metabolize and eliminate higher doses causing potentially toxic effects.

Fluoroquinolones are absorbed through the GI tract and metabolized and eliminated through the kidneys. Some factors that affect the pharmacokinetics of this antibiotic class include diet, pregnancy, and drug interactions. When taking fluroquinolones, patients should avoid “Dairy, antacids, multivitamins containing zinc, certain medications (eg, sucralfate, buffered formulation of didanosine), and other sources of divalent cations, [as they] can substantially decrease absorption” (Hooper, 2019). Pregnancy is also a major factor in determining appropriate antibiotic administration. Fluroquinolones are not safe for pregnant or lactating women as this class could harm the musculoskeletal development of a fetus or child (Hooper, 2019). Lastly, in combination with certain drugs, fluoroquinolones could potentially cause life threatening cardiac arrhythmias due to a prolonged QT interval (Hooper, 2019). As prescribers, nurse practitioners must thoroughly evaluate all factors prior to prescribing any class of drug.



Gordon, J., MD. (1998). SSRIs and St. John’s Wort: Possible Toxicity? American Family Physician Journal,1(57), 5th ser., 950-953. Retrieved from

Hirsch, M., MD, & Birnbaum, R. J., MD. (2020, March). Selective serotonin reuptake inhibitors: Pharmacology, administration, and side effects. Retrieved from serotonin reuptake inhibitors&source=search_result&selectedTitle=2~138&usage_type=default&display_rank=1#H399779781.

Hooper, D., MD. (2019). Fluoroquinolones. Retrieved from

Kim, I. H., Kisseleva, T., & Brenner, D. A. (2015). Aging and liver disease. Current opinion in gastroenterology31(3), 184–191.

Lode, H., Borner, K., & Koeppe, P. (1998). Pharmacodynamics of Fluoroquinolones. Clinical Infectious Diseases,27(1), 33-39.

Woo, T. M., & Robinson, M. V. (2016). Pharmacotherapeutics for advance practice nurse prescribers. Philadelphia: F.A. Davis Company.

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